Biofilm formation by algae as a mechanism for surviving on mine tailings

Photosynthetic biofilms successfully colonize the sediments of a mine tailings reservoir (Guanajuato, Mexico) despite the high metal concentrations that are present. To elucidate the mechanisms of biofilm survival despite metal ores, experiments were performed to evaluate the response of seminatural biofilms to Cu, Zn, and a combination of both metals at concentrations observed in the field. The biofilms were composed mostly of the chlorophyte Chlorococcum sp. and the cyanobacterium Phormidium sp., and their response to the two added metals was described by measurements of extracellular polymeric substances (EPS) and in vivo fluorescence. The photosynthetic efficiency and the minimal chlorophyll fluorescence of dark-adapted cells were measured by multiwavelength pulse amplitude-modulated fluorometry. The photosynthetic efficiency of light-adapted cells (Φ_PSII) also was measured. Metal exposure increased the EPS production of biofilms, as visualized with confocal laser-scanning microscopy. Extracellular polymeric substances enhanced the extracellular metal accumulation from the first day of metal exposure. Metals provoked changes in the relative abundance of the dominant taxa because of a species-specific response to the metals when added individually. Metals affected the Φ_PSII less than the total biomass, suggesting ongoing activity of the surviving biofilms. Survival of individual biofilm photosynthetic cells was found to resume from the embedding in the mucilaginous structure, which immobilizes the metals extracellularly. The survival of biofilms under mixed-metal exposure has practical applications in the remediation of mine tailings.